Deep Green Technology

Imagine that you are standing on a beach flying a kite in the wind, you feel a strong force from the kite in the rope and notice that the kite flies fast, way faster than the wind is blowing. If you would attach a turbine to the kite and put it in the ocean, where a water current flows instead of the wind blowing, you would have the concept or Deep Green, Minesto´s power plant. The secret behind Deep Green is that it reaches a speed 10 times higher than the water current, just like the kite flies faster than the wind. The speed has a cubic relationship to the power; ten times higher speed gives 1000 times more power.

Deep Green is currently undergoing long-term ocean trials in quarter scale in Strangford Lough, Northern Ireland. Read more about the technology development here.

The data sheets for the four Deep Green devices can be downloaded here.

Deep Green Producing Electricity

Deep Green produces electricity by a unique principle illustrated in the figure below. The water current creates a hydrodynamic lift force on the wing which pushes the kite forward (1). The kite is steered in an 8-shaped trajectory by a rudder and reaches a speed ten times the water current speed (2). As the kite moves, water flows through the turbine and electricity is produced in the gearless generator (3). The electricity is transmitted through a cable in the tether attached to the wing (4). The electricity continues in sub-sea cables on the seabed to the shore (5).

Deep Green´s Advantages

Site characteristics monopolyDeep Green is the only known power plant that cost-effectively produces electricity at sites with velocities between 1.2-2.5 m/s and depths between 60-120 meters.

High EfficiencyDue to the unique characteristic of increasing the relative flow Deep Green is the most cost effective power plant on the market today.

Small in size and lightweightThe plant weighs only seven tonnes which is 10-25 times less per MW than competing technologies which are designed for tidal flows above 2.4m/s.

Low-cost offshore operationsSmall boats and equipment are used for installation, service and maintenance. The offshore operations are minimised since only attachment and detachment has to done offshore.

Parts

Deep Green consists of a wing (1), which carries a turbine (2) directly coupled to a generator in a nacelle (3). A rudder (4), a servo and control system steers the kite in an 8-shaped trajectory. The struts (5) are connected to the tether (6) which attaches the kite to a swivel fastened to a foundation on the seabed. The tether accommodates power and communication cables.

The wing is designed to generate a high lift force when hit by the ocean current. The wing requires a stiff and lightweight structure with sufficient fatigue and material properties for 20 years’ service life. In addition the wing includes watertight compartments for buoyancy system, batteries and pressure sensors.

The tether is mainly a force bearer designed to take the high loads created by the wing, but it also accommodates power and signal cables. To improve performance the tether is designed considering hydrodynamic forces to reduce the drag and increase the electricity production.

The bottom joint mounted on the foundation is the anchoring point for the tether at the seabed and as such essential for the system assuring that the tether can move smoothly in all directions depending of the current velocity direction and the kite motions.

The power take-off system is where the electricity in Deep Green is produced. It consists of a generator driven by a tur­bine at the front of the nacelle. The electricity is transported from the generator, through the tether to the seafloor where it continues in a seafloor cable. Several power plants will be connected to each other in an array; a typical Deep Green array may consist of 100 devices supplying electricity for 33,000 households.

A word from the inventor

As an engineer I always have been interested in renewable energy and technical inventions. In year 2001, I was the project leader for a pre-study on long carbon fiber blade for large wind turbines at Saab AB (the aircraft manufacturer). In parallel I studied the economics of scale of wind turbines and different turbine concepts, especially vertical axis configurations, H-rotors and the Giromill concept from McDonnell Aircraft.

During this period a come across a study which predicted the weight for a MW-machine to hundreds of tonnes due to the need of large swept area and the low air density. Because of these limitations, I began to look for alternative solutions with lightweight construction. As carbon fiber is compatible with salt water and water is much denser than air (appr. 800 times) I found that the H-rotor machine would be smaller, more weight and cost efficient in tidal current. Then I realised, that the cross arm and tower could be even lighter if a high speed turbine and a generator was attached directly to the blade.

No mechanical moment was then transmitted through the cross arm and tower and no three stage gearbox was needed. In the next step, I replaced the cross arm and the tower with a wire attached to the bottom. This concept was possible if the blade or wing moved across the current all the time in a circular or an eight digit path, steered by a rudder (in the same way as the blades move on a horizontal axis wind turbine).

After discussions with hydrodynamic specialists the outcome was a compact, efficient tidal power plant able to sweep large areas, much more efficient than rotors on static structures, and the design offered a decrease in electricity generating cost. In year 2004, I presented the invention called the Enerkite (the former name of Deep Green) for Saab Ventures. During the following years, I supervised two students in their master thesis works on the Enerkite, which showed the potential of the invention theoretically and led to a proof of concept in sea tests. The thesis also led to the conclusion that the invention is highly suitable for lower-energy sites.